When i was considering buying the backpack accessory, i coudln't really find any information on it, so i thought i'd post my quick assessment of it. Sorry about the picture quality, i'm really bad at photography.

Here's a picture of it beside a Back Roller Classic:
The red x's are where the backpack accessory connects to the pannier.
At the top it uses the standard pannier hook thingies, and at the bottom, there's a slot on each end of the pannier, and an insert on the backpack accessory's straps, which slide in pretty tightly.

Here's a picture of it attached to the backpack.

And here's me making a stupid face with it on my back.

When i first took got it, and put it onto my pannier, i thought "huh, this is pretty cheap, i'm not very impressed", and everything seemed sort of "loose". But then i remembered i'm in no way qualified to judge a backpack accessory by looking at it, so i tried it out. It's as comfortable as any backpack i've used before. I used it on my first cycling trip and hiked around for 2 days with it relatively heavy, (it was carrying food and water), and it was comfortable the whole time.

I also don't have my top pannier clips in the default location, they're actually way more to one side, but that didn't seem to matter.

One slight annoyance is that it takes up a decent amount of room in the pannier when you're not using it, and it's a bit of an awkward shape because the of the straps and stuff.

One other annoyance which took me a day to remedy/get used to, is that if you lift it with the black strap on the pannier, it opens the top clips and which detaches it from the pannier. You have to lift it with the grey strap that's on the backpack accessory itself.

Pros:
Does what it's supposed to do.
Comfortable.
Comfortable with the pannier clips off center.

Cons:
Lifting with the wrong cable detaches it from the pannier at the top.
Takes up room in the pannier

I would recommend it to someone wanting the features it provides. I think i paid $30USD.

*disclaimer: I don't know much about backpacks, i've never done any sort of backpacking or anything.

I think the review is pretty much spot on. I have used it on my last tour in Europe.

I think it is quite annoying how much space this takes up in the pannier (although it is quite light). I guess that's the trade off for how comfortable it is. I was considering for my next tour just buying those cheap jansport backpacks (just a cheap regular backpack) that can be rolled up tightly and trying that instead. I am not sure about the different in comfort level but I think the jansport would pack smaller.

I've used my Ortlieb packpack accessory on a few tours. I just slide mine inside the pannier like a book without trying to roll it or fold it when I'm not using it, and arrange the straps so that they don't take up too much room. It eats up some space, but I still have plenty of room for all my other gear. I pack denser items in the pannier the adapter is in, to make up for the volume taken up by the adapter.

It is NOT as comfortable as my other day packs (Arc'Teryx, Black Diamond, CamelBack HAWG), but it's adequate for day hikes and sightseeing in cities when I'm not riding. It takes up less space in my pannier than a decent day pack would.

My opinion is that it is not a substitute for a good quality day pack, but it is an excellent way to turn a pannier into a daypack for casual use.

I took along an ultralite regular sized backpak. The kind I took uses your sleeping pad to maintain its shape. Mine was rated to carry lite loads ?25-30Lbs, plenty for a few days hike. It also worked very well loaded up with gear strapped on top of my rear rack. When not needed it rolled up pretty small, and it's empty weight was about 8oz.

Was also handy loaded as checked luggage (straps etc duct taped to not get torn off).

I just picked one of these up for use around the city mainly, but will likely use it for some bike/hike trips around the Bay Area. Your review is good, I just wish i would have read it before because i was looking at it attached to the pannier and thinking it was very loose. But everything tightens up when you put it on your back. It doesn't seem too uncomfortable. I think it will work well for what the reasons that i bought it.

In the instructions it show them rolling it up and clipping it with the top strap. Haven't tried it yet, but i will on my way home tonight. Once i get some more usage out of it i will post an update with my opinions.

EDIT: I meant to ask; if there is another way to remove the clips from the bottom of the pannier besides just pulling really hard?

I was playing around with the backpack accessory last night and realized that there are two extra loops at the bottom where you thread the strap that connects to the bottom of the pannier. These extra loops could be used to thread a some webbing with a buckle for a waist strap. I think that it is comfortable enough for day hikes as it is, as some stated not as comfortable as a good day pack, but with a waist strap I think it would be even better. Has anyone tried this?

I would still like to know if there is an easier way to disconnect it from the bottom of the pannier, I keep just pulling it really hard, and I feel like I am going to break something. Maybe I should just loosen the screw a half turn or so?

I coudln't figure out any other way to undue those straps either, unfortunately. It sounds like mine might not be as hard to take off as yours, so maybe loosening the screw would be a good idea, not sure though.